1. Field of the Invention
The invention relates to a method for checking the plausibility of an incremental counter, and to an evaluation device for checking the plausibility of the incremental counter.
2. Description of the Related Art
In order to detect digital pulses, incremental counters are used in measuring and control equipment. In a simple case, this equipment is composed of a mechanical or electronic counter module, where square-wave signals are usually detected with respect to rising or falling edges, or “zero crossovers”. An important application is the detection of the position or movement of machines, such as electric motors or driven axles of machine tools. In this context, it is often customary to detect what is referred to as a “quadrature signal”, via which it is also possible to detect a direction of movement, with the result that after an index switch is passed through once or a “zero position” is approached once, a counter reading of the incremental counter always provides information about the absolute position of an observed object or the attitude of a monitored rotor or the like. From the time reference between successive counting pulses, it is also possible to determine a parameter such as a clock rate, a movement speed, angular speed or rotational speed of an observed mechanical system.
If electrical interference (“interference pulses”) occurs on a signal line with which the counting pulses are fed to evaluation electronics or to an evaluation circuit, the evaluation electronics could incorrectly interpret this interference also as counting signals, as a result of which the counter reading of the incremental counter (“position value”) would be falsified and, moreover, the clock rate or the speed of the observed mechanical system would be detected incorrectly. The same applies if an actually triggered counting pulse is not detected by the incremental counter owing to an interruption or some other fault, as a result of which incorrect position information and, under certain circumstances, also an incorrect clock rate or speed of an observed mechanical system is also detected or measured.
The evaluation logic for incremental encoders, i.e., the incremental counters from the prior art, typically have filters that typically allow only counting signals below a specific frequency to pass through, where a maximum counting frequency or clock rate or speed is then automatically produced. Such filters can be implemented, for example, in an analogous manner by a correspondingly dimensioned RC element in the signal line, or else also as a digital filter, implemented, for example in a digital controller. In the case of rapid counting signals, in surroundings that are subject to a large amount of interference or when unshielded signal cables are used, it is possible, however, that pure filtering based on the signal frequency is not sufficient. Likewise, counting pulses that are suppressed erroneously, for example, due to brief interruptions in the signal line, cannot be found or suppressed because counting pulses that have been lost cause the signal frequency to be lowered. However, the described RC element or a corresponding digital filter constitutes a low-pass filter that is therefore not capable of detecting or filtering out such interference. In addition, it is possible in applications, for example, in the control of movements, which are programmed in stored-program controllers to perform plausibility checking of the detected speed in positioning systems, i.e., for example, to check whether a defined maximum speed or maximum clock rate has been exceeded. This plausibility checking is typically perform in the control clock of the stored-program controller, i.e., once in each cycle of the controller. In the case of customary cycle times of stored-program controllers, such as a millisecond, and the customary movement speeds of mechanical axles with a corresponding resolution of the signal pickups this means, however, that the counter reading of the incremental counters can change considerably from cycle to cycle, i.e., a multiplicity of counting pulses is detected between two cycles. It is therefore not possible to detect individual interference pulses that are erroneously interpreted as counting pulses if the proportion of interference pulses in the sum of detected counting pulses is comparatively low. Likewise, sporadic interruptions, which only suppress a comparatively small proportion of the actual signal pulses between two control cycles, likewise cannot be detected with this method.